Trump began the exchange accusing the network of teeing up questions for Bush and other candidates to bash the businessman.

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"I think it is very sad that CNN leads Jeb Bush, Governor Bush down a road by starting off virtually all the questions by ‘Mr. Trump this, Mr. Trump that,' " the party front-runner said during CNN's debate in Las Vegas.

He added most of the questions during CNN's first debate in August and Tuesday's undercard debate centered on him "just to get ratings," calling it "very unprofessional," to cheers from the audience.

Sensing another opportunity to punch at Trump, as has become his strategy, Bush struck back and chided the real estate magnate for thinking that he is being treated too tough.

"If you think this is tough and you aren't being treated fairly, imagine what it's going to be like dealing with [Chinese] President Xi, or dealing with the Islamic terrorism that exists. This is a tough business," Bush said.

The two then sparred back and forth, repeatedly trying to interrupt each other, with Trump mockingly saying to Bush, "You're a tough guy, Jeb."

But when Bush told Trump that he'd never be president by "insulting your way," Trump snapped back.

"I'm at 42 and you're at 3. So I'm doing better," Trump said to boos.

"You started off over here, Jeb," he added, pointing towards the center of the stage where the polling leaders stand. "You're moving over further and further; pretty soon you are going to be off the end."